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[Xen-devel] Re: [PATCH/RFC] report hardware tsc frequency even for emula

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH/RFC] report hardware tsc frequency even for emulated tsc
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 30 Nov 2009 13:06:44 -0800
Cc: "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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On 11/30/09 12:51, Dan Magenheimer wrote:
> As long as the "illusion" provided via both rdtsc and
> pvclock are consistent, it works.

OK, I see.  Rather than always emulating 1GHz, make it emulate some
other arbitrary speed depending on the initial native CPU speed.

> I wasn't aware of that.  My proposal could allow for that but
> the specific proposed patch doesn't.  It could only be
> done I think if one didn't care about the TSC hz rate
> given to apps... if that's true, just turn off TSC emulation.

I don't think its worth doing.  In many ways a VCPU is like a Transmeta
CPU: they never ran at any particular speed, but with some
power/performance policy setting.  The tsc ran at a fixed rate that had
no particular relationship to how fast instructions get executed.

> Good point.  OTOH, if one cared to do something to change
> tsc emulation, one would need to be privileged to relaunch
> the domain.

Well, they'd need some interface to start domains at all.  You could
imagine something like a VPS hosting service where the user can control
their own domains via a web GUI, and "tsc emulation" would be a checkbox
(buried deeping in an advanced options page, one presumes).

And you could also imagine that this patch would be a boon to them,
because it would eliminate the "you said you had 2.4GHz hosts but
/proc/cpuinfo only says 1GHz" questions/complaints...


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